Histoplasmosis is normally contracted by inhalation of the etiologic agent, Histoplasma capsulatum, from natural soil substrates where the fungus grows and reproduces as a saprophyte. A technique that concentrates spores from soil samples will be prefected to enable presumptive microscopic identification of infested soils and to obtain direct isolates of the fungus with passage through mice. Fluorescent antibody labelling will be used to obtain quantitative counts of Histoplasma units in soils and to locate the microhabitats where the fungus grows. Phenotypic variations will be determined, including morphological characters, mating type, serological reactions, ability to convert to the yeast (tissue) phase, and general level of virulence. A study of isozyme variability will be done to determine the genetic structure of natural populations of Histoplasma. Comparisons will be made of soils enriched with starling, chicken and bat excreta, and the level of genetic variation to be found within small samples, within populations, and between populations. Correlations between izozymic patterns and other phenotypes such as virulence will be sought.